=> R0 = 128
R1 = reason code
<= R0 preserved
R1 = bits 0-7 give LSB of answer
R2 = bits 0-23 give bits 8-31 of answer
This call returns the ADC conversion or checks the (buffered) mouse position or the free space in system buffers, depending on the reason code passed in R1. It is recommended to use OS_Mouse to read the mouse position, as this reads the current (unbuffered) position.
The ADC conversion was previously used on the BBC Microcomputer for the joystick and fire buttons. It is typically emulated by the joystick driver, but it is recommended to use Joystick_Read instead.
See PRM 1, page 166
OS_Byte 128 reason codes
entry R1 value returned
0 R1 = fire buttons
R2 = most recent ADC channel to complete, or 0 if no conversions running
1-4 R1 = low 8 bits of ADC conversion for that channel
R2 = high 8 bits of ADC conversion for that channel
5 Reserved for use with VFS (mouse x threshold)
6 Reserved for use with VFS (mouse y threshold)
7 mouse x
8 mouse y
9 Reserved for use with VFS (mouse buttons)
246 free bytes in mouse buffer
252 free bytes in printer buffer
253 free bytes in serial output buffer
254 free bytes in serial input buffer
255 free bytes in keyboard buffer
Note that for reading free space, R1 = 255 - buffer number.
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